In order to increase the market demand for wool during the Spring through Summer season, it would be desirable to increase the availability of products made from fine yarns and also to introduce some modification to their surface appearance, such as degree of lustre. Traditionally it has been necessary to use very fine wools of diameter less than 18 microns to enable fine yarns to be spun, and modification of surface appearance has required additional chemical treatment to that normally used. The chemical methods used to modify the surface appearance are degradative and result in loss of material. Some thinning of the fibre can be achieved in this way but typically a 5% change in diameter, say one micron, requires a 10% loss of material.
An alternative approach, at least in theory, is to stretch fibres so that their diameter is reduced to less than 18 microns. However, this approach has remained in the realm of theory and a successful commercial process is yet to be devised.
Attempts to stretch the fibres in an untwisted assembly such as a sliver, have to date required uneconomic and complex machinery: the fibres must be gripped substantially continuously or at intervals of about 50 to 70 mm over a substantial distance in order to achieve a residence time adequate to ensure setting of the stretch.
An example of this approach is described in British patent 1,189,994. The untwisted assembly is passed through an array of alternately oppositely laterally moving fibre grip devices. However, grip devices are required at 50 to 70 mm intervals and a treatment machine some 30 to 40 m long would be required to achieve an adequate residence time. Another technique has been described by Kim et al in Textile Research Journal, May 1984 at 325 and June 1984 at 370, in connection with the stretch mercerization of cotton fibres in roving form. This apparatus consists of a series of closely spaced drive rollers gradually increasing in diameter. Idler rollers were placed on top of the drive rollers and the roving was passed successively under the bottom rollers and over the top rollers. This arrangement could be adapted to treat a wool roving but a very large number of rollers would be required to achieve an adequate residence time if productivity at a commercial level is to be achieved.
British patent 1,196,419 proposes inserting twist into a sliver of staple fibres and then stretching the sliver. The twist increases the frictional engagement between the fibres to ensure that stretching of the fibres and not drafting of the sliver occurs. The method and apparatus described in this British patent require a device for inserting real twist upstream of a stretching arrangement comprising two longitudinally extending pairs of rollers about which the twisted sliver is wrapped and an untwisting device downstream of the stretching arrangement. In the apparatus of this patent the input twist insertion and output twist removal rates will be different and thus complex mechanical arrangements will be required to correlate the twist insertion and twist removal rates. If the twist removal is not exact, some residual twist will remain which will cause difficulties in further processing the sliver. Furthermore the insertion of real twist limits the apparatus to a batch mode of operation because the supply ball or wound assembly of sliver at the input end of the apparatus must itself be rotated to insert the twist. Productivity is therefore limited by the need to load a fresh supply ball or wound assembly of the sliver ready for the next run after the previous ball or assembly has been unwound. Although automation of such a batch system of processing to provide a continuous throughput system is conceivable, it would require further complex machinery at the input end thereby detracting from the commercial viability of the apparatus.
British patent 1,196,419 also discloses that a twist factor of between 600 to 1000 is required for performing the method. At such a high twist factor the sliver may snap when stretched before substantial stretch occurs given that the breaking strength of fibres steadily decreases as the twist factor is increased beyond a figure of about 150. Also such highly twisted slivers would be prone to "snarling" or self entanglement and therefore difficult to control.